Charge read-out structure for a photon / particle detector

ABSTRACT

A charge read-out structure for photon and particle detectors, which is capable of spatially-resolving a position of the charge. The structure comprises a resistive element defining a detection surface which is capacitively coupled to an array of electrically insulated electrodes. Each electrode in the array is capacitively coupled to an adjacent electrode in the array to form a capacitively coupled network of electrodes. Selected ones of the electrodes in the array are each coupled to an array output for connection to a respective charge measurement device. The resistive element has a resistivity sufficient to temporarily localize a charge induced on the resistive element to an area corresponding to a subset of said electrodes in the array and for a duration sufficient for signal measurement from the array of electrodes. Charge measurement devices are coupled to selected electrodes in the network such that the spatial position of a charge event in the network can be determined by comparing the outputs from each charge measurement device.

The present invention relates to charge read-out structures used in, forexample, photon and particle detectors, and in particular to chargeread-out structures capable of spatially-resolving a position of thecharge.

Detectors for detecting electromagnetic radiation quanta or particlesconventionally use a conversion device such as a photocathode togetherwith a microchannel plate electron multiplier to generate an electroncharge cloud arising from an interaction of the detector with aradiation quantum or particle to be detected. The electron charge cloudis detected and its position is spatially resolved using an adjacentanode structure.

U.S. Pat. No. 5,686,721 describes such a detector having aspatially-resolving anode structure comprising a high resistance,electrically conductive thin film anode adjacent to the chargemultiplier and a low resistance anode structure capacitively coupled tothe high resistance anode. The high resistance anode is formed on thevacuum side of a chamber wall and the low resistance anode is formed onthe other side of the chamber wall, e.g. at ambient atmosphericpressure. The chamber wall is formed of, e.g. glass, such that the highresistance anode and the low resistance anode are capacitively coupled.An electron cloud impinging on the high resistance thin film anoderemains there for a few tens of nanoseconds due to the high layerresistance. This capacitively couples through the glass layer of thechamber wall to generate an image charge on or in the low resistanceanode structure.

The low resistance anode comprises a spatially-resolving structure suchas a three-contact region wedge-and-strip arrangement. The spatiallocation of the image charge can be determined using a charge-sensitivepreamplifier for each contact region and an evaluation logic system.Other spatially resolving anode structures are described, such as aVernier anode, a spiral anode, a delay line layer and a pixel systemwith digital read out. The system of U.S. Pat. No. 5,686,721 requiresthat the internal resistances of the high- and low-resistance anodelayers are optimally matched to one another and that the geometricalconfiguration of the low-resistance anode is suited to spatiallyresolving the image charge. Spatial resolution may be limited by theconfiguration of individually readable contact regions of thelow-resistance anode.

R Gott et al: “The use of channel multiplier arrays for one and twodimensional x-ray image dissection”, IEEE Trans. Nucl. Sci., Vol. 17 (3)1970, pp. 367-373 describes a channel multiplier array and collectorsubsystem in which electron pulses from individual channels in the arrayare collected using an evenly spaced grid of wires. Adjacent wires inthe grid are coupled by condensers and each of the wires in the grid isresistively grounded. A charge pulse arriving at a particular wire inthe grid is divided by the capacitive network and fed to two chargesensitive pre-amplifiers. From these, the position of the charge pulse,relative to the grid, can be established. Spatial resolution may belimited by the number of discrete components, including the condensersand grounding resistors, that must be incorporated with the grid ofwires.

It is an object of the present invention to provide an improved,spatially resolving, charge read-out structure suitable for use in aphoton or particle detector.

According to one aspect, the present invention provides aspatially-resolving charge detection device comprising:

-   -   a resistive element defining a detection surface and being        capacitively coupled to an array of electrically insulated        electrodes, each electrode in the array being capacitively        coupled to an adjacent electrode in the array to form a        capacitively coupled network of electrodes,    -   selected ones of the electrodes in the array each being coupled        to an array output for connection to a respective charge        measurement device;    -   the resistive element having a resistivity sufficient to        temporarily localize a charge induced on the resistive element        to an area corresponding to a subset of said electrodes in the        array and for a duration sufficient for signal measurement from        the array of electrodes.

The charge detection device may include a plurality of chargemeasurement devices coupled to the network, each charge measurementdevice being coupled to a different electrode in the array to sample acharge therefrom. When used as a particle or photon detector, the chargedetection device may include a multiplication device for interactingwith the particle or photon and generating a charge cloud therefrom; themultiplication device being positioned adjacent to the resistive elementsuch that the resistive element interacts with the charge cloud tocapture a charge thereon. The multiplication device may be an electronmultiplication device. The multiplication device may be a microchannelplate, a photomultiplier, or a gas proportional counter. The interactingparticle may be a photon, ionizing particle or charged particle.

The array of electrically insulated electrodes may comprise a one, twoor three dimensional array of electrodes adapted for spatially resolvinga charge on the resistive element respectively in one or two dimensions.The array of electrically insulated electrodes may comprise an array ofelectrically conductive elements, with each element within the body ofthe array being capacitively coupled to the closest neighbour elementswith a first capacitance value, and each element at the periphery of thearray being capacitively coupled to adjacent elements on the peripheryof the array with a second capacitance value, the second capacitancevalue being greater than the first capacitance value. Each element inthe body of the array may be capacitively coupled to the next nearestelements in the array by a third capacitance value that is substantiallylower than the first capacitance value. The second capacitance value maybe between 10 and 100 times greater than the first capacitance value.

The charge measurement devices may be connected to selected elements inthe array, preferably elements at the periphery of the array and morepreferably corner elements of the array. A processing device may becoupled to each of the charge measurement devices to determine a spatialposition of a localised charge on the resistive element based on therelative outputs of each charge measurement device.

The array of electrically insulated electrodes may comprise a firstsub-array of electrically conductive elements, each element within thebody of the first sub-array being capacitively coupled to first selectedclosest neighbour elements with a first capacitance value, and eachelement in the body of the first sub-array having minimal or nocapacitive coupling with second selected closest elements in the firstsub-array, selected elements at the periphery of the first sub-arraybeing directly electrically connected to adjacent elements on theperiphery of the first sub-array. The array may include a secondsub-array of conductive elements complementary to, and overlapping thefirst sub-array, the first sub-array being adapted for detecting thespatial position of charge on the resistive element in one dimension andthe second sub-array being adapted for detecting the spatial position ofcharge on the resistive element in a second dimension different from thefirst dimension. Charge measurement devices may be connected torespective groups of the selected elements at the periphery of thearray.

The resistive element preferably has a surface resistivity adapted toenable charge localization over a time period in the range 1 to 1000 ns,or in other embodiments a time period in the range 1 to 10000 ns.

According to another aspect, the present invention provides a method ofspatially-resolving the position of charge on a resistive element of acharge detection device comprising the steps of:

-   -   capturing a charge on a detection surface defined by a resistive        element that is capacitively coupled to an array of electrically        insulated electrodes, each electrode in the array being        capacitively coupled to an adjacent electrode in the array to        form a capacitively coupled network of electrodes,    -   forming an capacitively induced charge on at least one of the        electrodes in the array;    -   measuring the capacitively induced charge by sampling the array        using a plurality of charge measurement devices coupled to        selected ones of the electrodes in the array;    -   determining the location of the capacitively induced charge        based on the relative outputs of said charge measurement        devices,    -   the resistive element having a resistivity sufficient to        temporarily localize a charge on the resistive element to an        area corresponding to a subset of said electrodes in the array        and for a duration sufficient for signal measurement from the        array of electrodes.

Embodiments of the present invention will now be described by way ofexample and with reference to the accompanying drawings in which:

FIG. 1 is a schematic perspective view of a photon detector with acapacitively coupled charge read-out structure;

FIG. 2 is a schematic circuit diagram of a capacitively coupled networkof elements for forming the charge read-out structure of a photondetector as in FIG. 1;

FIG. 3 is a perspective view of an actual device comprising acapacitively coupled network of elements for forming the charge read-outstructure of a photon detector as in FIG. 1;

FIG. 4 is a graphical representation of an image of a pinhole maskgenerated using a photon detector according to FIGS. 1, 2 and 3; and

FIG. 5 is a schematic circuit diagram of an alternative capacitivelycoupled network of elements suitable for forming the charge read-outstructure of a photon detector as in FIG. 1.

With reference to FIG. 1, a photon detector 1 comprises a photocathode 2or other suitable conversion medium for converting an incident photon 3or particle into an electron 4. Adjacent to the photocathode 2 is anelectron multiplier which, in this arrangement, comprises a microchannelplate 5. A first surface 11 of the microchannel plate 5 (the uppersurface as shown in FIG. 1) provides an input surface and a secondsurface 12 of the microchannel plate 5 (the underside as shown inFIG. 1) provides an output surface. Adjacent to the second surface 12 ofthe microchannel plate 5 is a planar resistive layer 7 formed on a firstsurface 13 of a dielectric substrate 8. On the opposing surface 14 ofthe dielectric substrate 8 is an array 9 of insulated electrodes 10which comprise a capacitively coupled network to be described further inconnection with FIG. 2.

In operation, an incident photon 3 interacts with the photocathode 2 ata particular event interaction x-y co-ordinate on the cathode, the x-yplane being defined by the plane of the photocathode. This interactiongenerates a photoelectron 4 which is accelerated toward and into themicrochannel plate 5 which is used to create a cloud of electrons 6 fromeach incident electron 4. The electron cloud 6 typically comprises 10⁵to 10⁷ electrons. The electron cloud emerges from the output surface 12of the microchannel plate 5 and is directed toward an anode formed bythe resistive layer 7. The cloud of electrons emerging from the outputsurface 12 of the microchannel plate 5 maintains spatial correlationwith the x-y event interaction co-ordinate and with x-y position of theincident photon at the input surface 11 because of the channel structureof the microchannel plate. The centroid of the charge cloud representsthe x-y coordinates of the incident photon 3.

The resistive layer 7 forms an anode for the photon detector and thepotential is defined by a surrounding conductor 16 which also provides aDC discharge path for charge collected on the anode. The resistive layer7 defines a resistive element having a resistivity that is sufficientlyhigh to temporarily localize a charge generated by the electron chargecloud 6 to an area of the anode for sufficient period of time for signalmeasurement from the array 9 of insulated electrodes 10. Preferably, theresistivity is such that the charge remains localized to a firstinsulated electrode 10 in the array 9 (or several adjacent electrodes10) for a suitable sampling period. A suitable sampling period may bebetween 1 and 10000 ns. The resistivity of the resistive layer 7 issufficiently low that the charge leaks away via the surroundingconductor 16 over a longer timescale than the sampling period. A rangeof possible resistivity values for the resistive element is 100 kOhm to100 MOhm per square, and optimally between 1 MOhm to 10 MOhm persquare). In this way, an image of the spatial distribution of theincident photons 3 arriving at the photocathode 2 can be generated.

An exemplary arrangement for the array 9 of insulated electrodes 10 isshown schematically in FIG. 2. The electrodes 10 generally form a twodimensional array of electrically conductive elements in which eachelement is capacitively coupled to its immediate closest neighbours.Thus, in the example of element 22, it is capacitively coupled to itsfour closest neighbour elements as indicated schematically by firstcapacitances 23 a, 23 b, 23 c, 23 d disposed parallel to the array axes.More particularly, each conductive element 22 is also capacitivelycoupled to each next nearest element in the array as indicatedschematically by second capacitances 24 a, 24 b, 24 c, 24 d disposeddiagonally to the array axes. This is the configuration for all elements22 that are disposed within the body of the array 9, i.e. that havenearest neighbour elements on all four sides. The elements in the bodyof the array are generally numbered 21.

For the elements 20 at the periphery of the array, i.e. those not havinga nearest neighbour element on all four sides, these elements arecapacitively coupled to each other with a capacitance value that isgreater than the capacitive coupling between elements within the body ofthe array. For example, peripheral element 25 is capacitively coupled tonearest neighbour peripheral elements 25 a, 25 b by capacitancesindicated schematically as third capacitances 26 a and 26 brespectively. Peripheral element 25 is also capacitively coupled tonearest neighbour 25 c within the body of the array by a fourthcapacitance indicated by capacitor 27.

Preferably, the third capacitances 26 (i.e. capacitance values couplingadjacent elements 20 at the periphery of the array 9) are between 10 and100 times greater than the first capacitances 23 (i.e. capacitancevalues coupling closest neighbour elements 21 within the body of thearray), or any suitable value that facilitates a linear response as willbe discussed hereinafter. The first capacitances may be as low as 1 pFor less. The second capacitances are preferably as low as possible tominimize non-linearity. In a preferred embodiment, the secondcapacitances are less than 10% of the first capacitances.

A charge measurement device 28A, 28B, 28C, 28D is provided at eachcorner of the array 9, being connected to the corner elements 20 of theperiphery of the array. Preferably, the charge measurement devices 28comprise low impedance amplifiers which may be charge sensitive ortrans-impedance amplifiers.

The capacitive coupling of the electrodes 10 in the network results in adivision of the capacitively induced charge among the elements 20, 21 inthe network in a manner that is defined by the particular networkconfiguration and which can be detected by the charge measurementdevices 28A to 28D which serve as read-out nodes.

FIG. 3 shows a perspective view of an exemplary physical arrangement ofan array 9 of capacitively coupled electrodes 10. The array showncomprises an array of insulated electrodes. Each electrode comprises aconductive area on a first side of the substrate electrically coupled toa corresponding conductive area on the second side of the substrate byway of a via. The conductive areas on the first side of the substrateare insulated from one another, and the conductive areas on the secondside of the substrate are also insulated from one another. Theconductive areas on the second side (those visible in the figure) areused for connection of the charge measurement devices and surfacemounted peripheral capacitors 30 added around the perimeter to providethe higher capacitance needed between the peripheral elements 25 in thearray 9. These correspond to the third capacitors 26 a and 26 b of FIG.2. The four charge signals from the array are fed to the chargemeasurement devices 28A . . . 28D (FIG. 2) via the wires 31 shown inFIG. 3. FIG. 3 shows the rear side of the device, i.e. thatcorresponding to the surface 14 in FIG. 1. The other surface of theboard not seen in FIG. 3 is held in contact with the dielectricsubstrate 8 of the detector 1.

A real image taken with a microchannel plate detector in photon-countingmode is shown in FIG. 4. An image of a 1 mm pitch pinhole mask was takenusing the photon counting detector of FIG. 3 with the read-out anodearray 9 configuration shown in FIG. 2. The active area of the imagereadout is 25 mm. The event count integrated over a period of time as afunction of x-position is plotted in the graph along the x-axis; theevent count integrated over a period of time as a function of y-positionis plotted in the graph along the y-axis; and the resulting x-yintensity plot is shown with intensity grey scale on the right handside. The mask comprised a square array of pinholes with a 1 mm pitchplus fours arrays of pinholes arranged as arcs in the four corners ofthe mask (of which only two and a half are visible due to misalignment).

The exemplary embodiment shown in FIG. 1 comprises a photon or particledetector 1 using a microchannel plate 5 as an electron multiplicationdevice. Alternative arrangements may include a detector using an imagingphotomultiplier or gas proportional counter. The dielectric substrate 8may be any suitable material for capacitively coupling the resistivelayer 7 with the array 9 of insulated electrodes 10. The array 9 ofelectrodes 10 and the resistive layer 7 may be constructed on oppositesurfaces of the same dielectric substrate 8 or they may be constructedas separate elements placed in proximity to one another. The latterconfiguration has manufacturing advantages when applied to vacuumphotomultiplier tube applications since the array 9 and connectionsthereto need not meet the vacuum tube material specification andprocessing requirements.

The signal charge on the electrode array 9 is capacitively induced andis AC coupled, so no resistive coupling of these electrodes is requiredfor discharge. A simple array of insulated conductive elements 10 (e.g.insulated islands of copper on a single or multi-layer PCB) can be used,such as shown in FIG. 3.

The physical capacitance of the geometry of the electrodes 10 can beused in the bulk of the pattern for the array coupling, greatlysimplifying the design by avoiding the need for discrete passivecomponents (capacitors). The preferred design shown in FIG. 3 usessurface mount capacitors 30 between each perimeter electrode 25, butother designs can either use enlarged area perimeter conductors toproduce these higher capacitance values, or do not require suchcapacitances.

The preferred design of FIG. 2 is a two dimensional array of conductorelements 20, 21 each capacitively coupled to nearest and second nearestneighbouring electrodes. In this scheme the charge is preferablymeasured at the four corners of the array (as shown by measurementdevices 28A to 28D. More generally, the charge measurement devices maybe connected to selected elements in the array, preferably at peripheraledges of the array, and more preferably the corner elements of thearray. A higher capacitance is preferred between neighbouring electrodes25 around the perimeter of the array (typically 10-100 times the nearestneighbour capacitance) to produce a linear response. The decodingalgorithm is:

x=(Q _(A) +Q _(B))/(Q _(A) +Q _(B) +Q _(C) +Q _(D))

y=(Q _(A) +Q _(D))/(Q _(A) +Q _(B) +Q _(C) +Q _(D))

where x and y are the charge centroid coordinates.

An alternative configuration for the array 9 of conductor elements 10 isshown in FIG. 5. The configuration of array 50 has a first sub-arraycomprising vertical chains 51 of conductor elements 10, the verticalchains 51 all connected at one end to charge measurement device 58B andall connected at the other end to charge measurement device 58D. Thearray 50 has a second sub-array comprising horizontal chains 52 ofconductor elements 10, the horizontal chains 52 all connected at one endto charge measurement device 58A and all connected at the other end tocharge measurement device 58C. The two sub-arrays preferably have no orminimal inter-capacitance. There is preferably no or minimal capacitancebetween elements 10 from adjacent chains within the same sub-array, asshown schematically. The capacitive division in this configuration ofarray is intrinsically linear and no additional, higher valuecapacitances are needed at the periphery of the array. However, divisionof the initial charge signal between the two sub-arrays may reduce thesignal-to-noise ratio, and hence may degrade position resolutionslightly.

Thus, in a general aspect, the charge can be divided into two separatesub-arrays of linear capacitance dividers. This configuration has anintrinsically linear response but somewhat lower spatial resolution,since only half of the signal is used to determine each positioncoordinate. The decoding algorithm for this array is:

x=(Q _(A))/(Q _(A) +Q _(C))

y=(Q _(B))/(Q _(B) +Q _(D))

Other designs of array can be considered which use more than fourmeasurement nodes distributed in a two dimensional array within theactive imaging area. A signal event charge can be collected on a subsetof the measurement nodes, allowing the other temporally overlapping orsimultaneous events to be detected on other node subsets.

The signal charge at each node is electronically measured, for exampleusing a charge sensitive preamplifier, shaping amplifier and analogue todigital converter. Other techniques using fast timing preamplifiers,discriminators and the time-over-threshold techniques to measure pulseheight (equivalent to charge) may be considered.

Using the intrinsic capacitance of the electrode array 9 of FIG. 3(rather than discrete components added to the array except at theperiphery thereof) can provide a major performance advantage since itreduces the capacitive load on the measurement devices (compared to theuse of discrete components). This reduces the noise, and improves thespatial resolution by a large factor or, conversely, allows the detectorto operate at lower gain which is a distinct advantage at high countrates. The combination of low noise capacitive division with the chargelocalisation provided by the resistive layer 7 reduces the noise furtherby eliminating partition noise. Partition noise is a noise contributionresulting from the statistics of the division of discrete charges (inthis case, electrons) amongst a number of electrodes. Since we are notdirectly collecting, but capacitively coupling, the collected charge(which is temporarily localized on the resistive layer 7) to theelectrodes 10, this noise component may reduce to zero. Partition noise,when present, dominates at low signal levels, owing to itsproportionality to the reciprocal of the square root of collectedcharge, so the lack of this noise component also benefits performance atlower detector gain.

The device and method described here can be used in all fields wherehigh resolution photon-counting, imaging is required in conjunction withprecise photon arrival timing, of the order of picoseconds. Such fieldsinclude: (a) time resolved spectroscopy—FLIM, FRET, FCS, single moleculeimaging; (b) 3D imaging, range finding, LIDAR, CRDS, CEAS; (c) quantumimaging, time correlated event imaging; (d) time of flighttechniques—mass spectrometry, field ion microscopy, molecular dynamics;(e) neutral beam imaging—MBE diagnostics; (f) optical diffusiontomography; (g) luminescence/phosphorescence; (h) picosecond detectorswith multi-pixel readout for particle physics, high energy physics, andastroparticle physics experiments e.g. for optical readout of fastscintillators in Cherenkov detectors.

Various changes to the illustrative embodiments are possible withoutdeparting from the scope of the invention. Although the resistive layer7 and array 9 have been shown as having planar configuration, it will beunderstood that a non-planar geometry such as concave or other form ofcurve could be considered while still resolving charge events in twodimensions over the surface of the layer. It will also be understoodthat the principles can be used with a one dimensional array, i.e.resolving charge events in one dimension. It will also be understoodthat the principles can be used with a three dimensional array, withseveral stacked, insulated planes of electrodes using electrode overlapbetween the layers to produce inter-electrode capacitances of a suitablevalue. Such three dimensional arrays of electrodes could be realizedusing the planes of conductors within a multi-layer printed circuitboard. In all of the examples discussed above, the arrays of electrodesneed not be rectangular nor even strictly regular or periodic, providedthat the location of charge events can still be determined by anappropriate algorithm that takes into account the array geometry.

Other embodiments are intentionally within the scope of the accompanyingclaims.

1. A spatially-resolving charge detection device comprising: a resistiveelement defining a detection surface and being capacitively coupled toan array of electrically insulated electrodes, each electrode in thearray being capacitively coupled to an adjacent electrode in the arrayto form a capacitively coupled network of electrodes, selected ones ofthe electrodes in the array each being coupled to an array output forconnection to a respective charge measurement device; the resistiveelement having a resistivity sufficient to temporarily localize a chargeinduced on the resistive element to an area corresponding to a subset ofsaid electrodes in the array and for a duration sufficient for signalmeasurement from the array of electrodes.
 2. The charge detection deviceof claim 1 further including a plurality of charge measurement devicescoupled to the network, each charge measurement device being coupled toa different electrode in the array to sample a charge therefrom.
 3. Thecharge detection device of claim 1 for use as a particle or photondetector, further comprising: a multiplication device for interactingwith a particle or photon and generating a charge cloud therefrom; themultiplication device being positioned adjacent to the resistive elementsuch that the resistive element interacts with the charge cloud tocapture a charge thereon.
 4. The charge detection device of claim 1 inwhich the array of electrically insulated electrodes comprises an atleast two dimensional array of electrodes adapted for spatiallyresolving a charge on the resistive element in two dimensions.
 5. Thecharge detection device of claim 4 in which the array of electricallyinsulated electrodes comprises an array of electrically conductiveelements, each element within the body of the array being capacitivelycoupled to the closest neighbour elements with a first capacitancevalue, each element at the periphery of the array being capacitivelycoupled to adjacent elements on the periphery of the array with a secondcapacitance value, the second capacitance value being greater than thefirst capacitance value.
 6. The charge detection device of claim 5 inwhich each element in the body of the array is capacitively coupled tothe next nearest elements in the array by a third capacitance value thatis substantially lower than the first capacitance value.
 7. The chargedetection device of claim 5 in which the second capacitance value isbetween 10 and 100 times greater than the first capacitance value. 8.The charge detection device of claim 5 further comprising a plurality ofcharge measurement devices each connected to a selected element at theperiphery of the array.
 9. The charge detection device of claim 8 inwhich the selected elements are corner elements.
 10. The chargedetection device of claim 1 in which the array of electrically insulatedelectrodes comprises a first array of electrically conductive elements,each element within the body of the first array being capacitivelycoupled to first selected closest neighbour elements with a firstcapacitance value, and each element in the body of the first arrayhaving minimal or no capacitive coupling with second selected closestelements in the array, selected elements at the periphery of the firstarray being directly electrically connected to adjacent elements on theperiphery of the first array.
 11. The charge detection device of claim10 further comprising a second array of conductive elementscomplementary to, and overlapping the first array, the first array beingadapted for detecting the spatial position of charge on the resistiveelement in one dimension and the second array being adapted fordetecting the spatial position of charge on the resistive element in asecond dimension different from the first dimension.
 12. The chargedetection device of claim 10 further comprising a plurality of chargemeasurement devices each connected to a group of said selected elementsat the periphery of the first or second arrays.
 13. The charge detectiondevice of claim 1 in which the array of electrodes is a substantiallyrectangular array.
 14. The charge detection device of claim 3 in whichthe multiplication device comprises an electron multiplication device.15. The charge detection device of claim 14 in which the multiplicationdevice comprises a microchannel plate, a photomultiplier, or gasproportional counter.
 16. The charge detection device of claim 1 inwhich the interacting particle can be a photon, ionizing particle orcharged particle.
 17. The charge detection device of claim 1 in whichthe resistive element has a surface resistivity adapted to enable chargelocalization over a period in the range 1 to 10000 ns.
 18. The chargedetection device of claim 2 further including a processing devicecoupled to each of the charge measurement devices to determine a spatialposition of a localised charge on the resistive element based on therelative outputs of each charge measurement device.
 19. A method ofspatially-resolving the position of charge on a resistive element of acharge detection device comprising the steps of: capturing a charge on adetection surface defined by a resistive element that is capacitivelycoupled to an array of electrically insulated electrodes, each electrodein the array being capacitively coupled to an adjacent electrode in thearray to form a capacitively coupled network of electrodes, forming ancapacitively induced charge on at least one of the electrodes in thearray; measuring the capacitively induced charge by sampling the arrayusing a plurality of charge measurement devices coupled to selected onesof the electrodes in the array; determining the location of thecapacitively induced charge based on the relative outputs of said chargemeasurement devices, the resistive element having a resistivitysufficient to temporarily localize a charge on the resistive element toan area corresponding to a subset of said electrodes in the array andfor a duration sufficient for signal measurement from the array ofelectrodes.